1. Field of the Invention
The invention pertains generally to internal combustion engines utilizing gaseous fuels, and more particularly to a method and apparatus for operating an internal combustion engine at high efficiency from an arbitrary mixture of multiple gaseous fuels such as hydrogen and natural gas.
2. Description of the Background Art
The current use of fossil fuels, such as gasoline, diesel fuel, and natural gas, to power various forms of internal combustion engines, in particular those incorporated within motor vehicles, has a number of serious shortcomings in view of dwindling fossil fuel resources and the increasing awareness of the detrimental effects of pollution. The desire to enjoy abundant energy while striving for the benefits of clean air have led to the consideration of various alternative energy sources for powering equipment such as motor vehicles. The use of renewable forms of energy is highly preferred to assure that energy remains abundant despite dwindling fossil fuel resources.
In recent years, the desire to use clean, renewable, vehicle energy sources has been evidenced by a push toward the use of electrical vehicles. The adoption of electrical vehicles, however, has proceeded slowly and a number of electric vehicle manufacturers have discontinued sales. Despite the enormous expenditures to develop electric vehicles and recharging equipment, the fundamental shortcomings of the technology and infrastructure have never been overcome. It should be appreciated that, although electrical energy may be readily converted to mechanical energy without generating high emission levels, electrical energy storage within batteries has many inherent drawbacks, including the time required to recharge a battery, the cost of batteries, and the weight per unit of energy stored within a battery. In contrast, conventional internal combustion engines (ICE) powered from liquid or gaseous fuels may be readily “recharged” by refueling, while the fuels themselves provide about a thirty-fold increase in energy storage density when compared with battery energy sources. However, the drawbacks associated with emissions and other environmental concerns, as well as the non-renewable nature of these fossil fuels, remain.
In response to these concerns, a number of alternative fuels have been considered to reduce air-borne emissions while maintaining the convenience and energy storage efficiency that are inherent within a combustion process. Increasingly, attention is being focused on hydrogen as a fuel for use within both internal combustion engines (ICE) and fuel cells. When utilized within an ICE, it should be appreciated that hydrogen provides a clean burning renewable energy source that may be readily produced. Home hydrogen refueling appliances have been proposed for use with hydrogen vehicles which are small in size and capable of generating sufficient hydrogen to power a vehicle for a trip spanning a few hundred miles. Vehicles incorporating hydrogen powered internal combustion engines have been studied and have been found to provide significant benefits from lowered emission levels and fuel renewability.
On-board vehicle energy reforming is also being considered, wherein a hydrogen powered vehicle is provided with a fuel reformer that converts the available fossil fuel to hydrogen gas which is utilized to operate a hydrogen combustion engine or a hydrogen fuel cell. A number of disadvantages exist, however, with regard to the adoption of on-board reforming for the purposes of facilitating the introduction of vehicles which operate from hydrogen fuel cells.
The adoption of hydrogen as an energy source has been a slow process, perhaps due in part to the inherent difficulty of changing an existing infrastructure to accommodate the use of hydrogen. The present infrastructure is lacking in both vehicles and fueling facilities that are capable of using, or distributing, hydrogen. Changing the present infrastructure to provide hydrogen distribution while synchronously developing and deploying hydrogen-fueled vehicles is a formidable challenge. It will be appreciated that vehicle manufacturers are resistant to invest in the development and marketing of hydrogen vehicles until the fuels are readily available, while fuel manufacturers are resistant to invest in widespread hydrogen production and distribution facilities until vehicles exist for consuming hydrogen fuels.
On the other hand, natural gas is a widely distributed form of gaseous hydrocarbon fossil fuel that typically comprises methane, although proportions of ethane, propane, and butane may also he present. Presently, an infrastructure exists to distribute natural gas for use in many applications, including motor vehicles. It should be appreciated that, at one point in recent history, automobiles and fuel distribution facilities were being rapidly adapted for the use and distribution, respectively, of natural gas because the prices of natural gas were well below that of gasoline and the conversion process was inexpensive. The transition from natural gas to hydrogen gas may appear trivial in that vehicles configured for burning natural gas may be reconfigured to exclusively burn hydrogen gas. One key difference between these fuels, however, is the energy density contained per cubic foot. Natural gas provides a significantly higher energy density than hydrogen gas, and consequently, an engine configured to operate on hydrogen gas, instead of natural gas, requires a higher fuel volume per combustion cycle to deliver a given rated horsepower and torque. Therefore, it will be appreciated that combustion variables must be reconfigured to provide for the burning of hydrogen gas. Consequently, the adoption of hydrogen gas as a “replacement” for natural gas would not solve the inherent infrastructure problems associated with the introduction of a new incompatible fuel source, and the conversion of vehicles to hydrogen could only be expected after an adequate hydrogen fuel distribution network had been established.
The use of hydrogen as an energy source within internal combustion vehicles has additional aspects that should be appreciated. First, although the hydrogen combustion process involves burning increased volumes of gaseous fuel, the resulting emissions contain lower levels of air pollutants and carbon dioxide than a comparable engine generating a give horsepower when operating from a natural gas fuel source. Second, hydrogen is a renewable fuel source that may be generated from a number of low-cost processes, whereas natural gas is a limited fossil fuel resource. It is anticipated, therefore, that the cost of fossil fuels will increase as the supply dwindles and that the low cost of generating renewable hydrogen will become increasingly attractive. The widespread use of hydrogen in future vehicles should then result in a lower cost per mile than that obtainable through the use of non-renewable fossil fuel resources.
Despite the inherent emission and renewability advantages of hydrogen use, the implementation of hydrogen vehicles has proceeded slowly. It will be appreciated that convenient operation of a hydrogen vehicle over distances requiring refueling has not been possible thus far due to a lack of hydrogen fuel distribution facilities. Experimental hydrogen vehicles are therefore only capable of operating within a limited commute radius about a facility-based refueling station.
The deployment of a substantial number of vehicles capable of operating from hydrogen would be an impetus for establishing the necessary elements of a hydrogen-fueling infrastructure. An internal combustion engines fueled by pure hydrogen would be capable of operating over a large range of fuel-air mixture ranges, including operation at a fuel-air mixture that is as lean as one fourth of stoichiometric fuel. It will be appreciated that in order for the hydrogen to he completely consumed a specific volume of air is required. The combustion process is generally given by the following formula:2H2+(O2+3.77N2)=>2H2O+3.77N2  (1)As a result of the combustion process, a total of 4.77 volumes of air are required for every 2 volumes of hydrogen utilized in the combustion process. This ratio is commonly referred to as a stoichiometric mass ratio of 34.3:1.
Optimal efficiency with a hydrogen ICE is attained at equivalence ratio φ of about 0.4, whereas a natural gas engine typically operates slightly lean of stoichiometric. By way of reference, it will be appreciated that the equivalence ratio is defined as the fuel/air mixture ratio normalized by the stoichiometric fuel/air mass. Efficiency is extremely important for hydrogen fueled vehicles, as the available fuel storage volume limits vehicle range. The hybrid electric configuration is therefore attractive for hydrogen fueled vehicles due to the ability to set up the engine to operate at substantially constant speed at peak efficiency consistent with low emissive output. Furthermore, the flame speed of a hydrogen-air mixture is substantially above that of a mixture of natural gas and air, although the speeds are decreased as the mixtures are made more lean. Fixed mixtures of hydrogen and natural gas have been developed, most notably Hythane™ which is a mixture of natural gas containing about 15% hydrogen, wherein the hydrogen contributes approximately 5% energy of the combustion energy. The Florida Solar Energy Center has experimented with mixtures having fixed percentages of hydrogen and methane. These mixtures utilize specific ratios of hydrogen up to about 36%, by volume, within the natural gas to contribute up to approximately 12% of the gaseous fuel energy. Researchers noted that the addition of hydrogen to natural gas aids lean operation and clean burning, and that either pure hydrogen or 30% hydrogen/natural gas can fuel an ICE so as to meet the EZEV (Equivalent Zero Emissions Vehicle) standard.
This body of work on the use of hydrogen fuel, and fuels containing hydrogen, indicates that efficient combustion may occur with natural gas, hydrogen, or a specific mixture thereof. However, because the fuel metering and timing of a vehicle is determined by the fuel being utilized, these fuels require that the engine be designed or specifically configured for use with a chosen fuel mixture. This imposes significant limits on the fuel mixtures that can be employed.
Therefore, a need exists for equipment and methods to ease the transition from conventional fossil based fuels to the widespread adoption of hydrogen fuel. The present invention satisfies that need, as well as others, and overcomes the deficiencies of previously developed vehicle energy solutions.